There is an ever increasing demand for sustainable, clean and low-cost sources of electrical energy. Though predating the invention of the internal combustion engine, hydrogen fuel cell technology has faced several major obstacles that have prevented its mainstream adoption as an electricity and power source. The main obstacles preventing further implementation of fuel cells as a main power source include the expense of the required materials and the fragility and failure of the components making up the fuel cells.
Precious metals are conventionally used in aqueous hydrogen fuel cells as a catalytic electrode material to lower the overpotential required for hydrogen oxidation. For example, platinum can be used as a catalytic electrode material for both half-cell reactions of a hydrogen fuel cell (hydrogen oxidation and oxygen reduction). However, the expense and rarity of these metals has hindered the widespread adoption of hydrogen fuel cell technology. As a result, there have been many studies aimed at finding abundant, inexpensive alternative catalysts to precious metals that lower the overpotential required for hydrogen oxidation.
The majority of these studies have focused on the use of natural and synthetic hydrogenase enzymes with [FeFe] or [FeNi] metal-centred active sites having suitable pendant Lewis basic ligands in close proximity (for example P. M. Vignais et al; Chem. Rev. 2007, 107, 4206-4272). These enzymes are able to overcome the high level of energy required to heterolytically cleave hydrogen by virtue of the strong hydricity of the metal centre and the strong proton acceptors provided by the pendant Lewis base ligands.
Research has also been conducted into the use of molecular electrocatalysts containing nickel, iron, cobalt and molybdenum metal centres. Rauchfuss et al (M. R. Ringenberg et al; J. Am. Chem. Soc.; 2008, 130, 788-789) have provided an alternative approach to the electrocatalytic oxidation of hydrogen using unsaturated iridium complexes with redox-active non-innocent amidophenolate ligands.
All of these previous approaches still require a metal-containing catalyst and there is a prevalent research focus on enzymes and synthetic biomimetic electrocatalysts for proton reduction to generate hydrogen, rather than the oxidation of hydrogen. Thus far, the challenge of finding an inexpensive, metal-free method of oxidising hydrogen has not been solved by this area of research.
It is therefore an object of the present invention to provide a catalyst system for a fuel cell which can reduce the overpotential required for hydrogen oxidation without the need for expensive and fragile metal catalysts systems. For example, a fuel cell which reduces or removes the need for a precious metal or biomimetic metal-centred enzyme type catalysts would reduce the expense and also eliminate the risk of metal catalyst contamination or poison.
An alternative method of heterolytically cleaving hydrogen has been suggested by Stephan et al (G. C. Welch et al; Science, 2006, 314, 1124-1126). The method requires the presence of a sterically encumbered Lewis acid and Lewis base that are incapable of forming a classical Lewis adduct due to the hindrance of sterically bulky groups. The Lewis acid and Lewis base are said to form a frustrated Lewis pair (FLP) which, on the introduction of hydrogen into the system, is able to heterolytically cleave the hydrogen molecule to form a hydride of the Lewis acid and a protonated base.
It is known in the art that boranes can be used as the Lewis acid component and phosphines or amines can be used as the Lewis base component for the frustrated Lewis pair, although these components can be combined on the same molecule. Typically these components will comprise sterically bulky groups that hinder the formation of a dative covalent bond. Examples of such frustrated Lewis pairs include tBu3P/B(C6F5)3, 2,2,6,6-tetramethylpiperidine/B(C6F5)3 and (Mes)2P(C6F4)B(C6F5)2.
Known applications for frustrated Lewis pairs include their use as metal-free hydrogenation catalysts and also using the borane hydride adduct as activating or reducing agent for other small molecules such as imines, enamines, nitriles or CO2.
A process for the catalytic hydrogenation of a variety of organic substrates using a frustrated Lewis pair catalyst is described in WO2013/177708 A1. The frustrated Lewis pair is a carbene stabilised borenium complex combined with a substituted amine or phosphine, which is used as a catalyst in chemical hydrogenation and reduction reactions.
In addition to the above uses, Stephan et al. (Chem. Comm; 2009, 1118) describes the use of a ferrocene redox label attached to an FLP to observe the reduction of the proton on FLP-activated mono- and bis-ferrocenylphosphines, in addition to observing the ferrocene signal itself.
However, there have been no investigations to date of the electrocatalytic behaviour of frustrated Lewis pairs or the tuning of these systems to create a clean and efficient source of energy from the heterolytic cleaving of dihydrogen (H2). In addition, these frustrated Lewis pair systems have not been employed to reduce the overpotential required to oxidise hydrogen in hydrogen fuel cell applications.